Champion costumes:
Beat the game in championship mode to unlock the champion costumes.

Extra characters:
To unlock... ...you have to:
Freak E. Deke Beat the game in arcade mode.
Michael Jackson Beat the game in arcade mode twice.
G.C. Thunder Beat the game in arcade mode three times.
Wild "Stubby" Corley Beat the game in arcade mode four times.
Shaquille O'Neal Beat the game in arcade mode five times.
Freedom Brock Beat the game in arcade mode six times.
Rocket Samchay Beat the game in arcade mode seven times.
Robox Rese-4 Beat the game in arcade mode eight times.
Mr. President Beat the game in arcade mode nine times.
The First Lady Beat the game in arcade mode ten times.

if you cannot download trainer then go to Chrome/Firefox's settings, specifically to the Privacy settings and simply unchecking the "Enable phishing and malware protection" option.

Some Game Trainers are sometimes reported to be a Virus or Trojan,
the most common is a keylogger called HotKeysHook or the file
has been packed/protected with VMProtect or Themida and
is recognized as Win32/Packed.VMProtect or Win32/Packed.Themida.
In ALL cases this is a FALSE ALARM as NONE
of the Game Trainers @ GCW contain known malicious code! More info in
the PC
Games FAQ!